The invention relates to compositions and methods for treating subterranean formations. More particularly, it relates to compositions and methods for treating a subterranean formation penetrated by a well bore into which a gel with a high viscosity is injected.
Viscous well treatment fluids are commonly utilized in the drilling, completion, and treatment of subterranean formations penetrated by well bores. A viscous well treatment fluid is generally composed of a polysaccharide or synthetic polymer in an aqueous or non-aqueous solution, which is cross-linked by an organometallic compound or a metal source compound. Examples of well treatments in which metal-crosslinked polymers are used include hydraulic fracturing, gravel packing operations, water blocking, and others.
Hydraulic fracturing techniques are widely employed to enhance oil and gas production from subterranean formations. During hydraulic fracturing, a proppant-laden fluid is injected into a well bore under pressure. Once the natural reservoir pressures are exceeded, the fracturing fluid initiates a fracture in the formation that generally continues to grow during pumping. The treatment design generally requires the fluid to reach a maximum viscosity as it enters the fracture which affects the fracture length and width. Adding cross-linking agents can further increase the viscosity of the fracturing fluid. The gelled fluid may be accompanied by a propping agent (i.e., proppant) which results in placement of the proppant within the fracture thus produced. The proppant remains in the produced fracture to prevent the complete closure of the fracture and to form a conductive channel extending from the well bore into the formation being treated once the fracturing fluid is recovered.
Gelled hydrocarbons have been used in petroleum producing subterranean formations as fracturing fluids to improve the recovery of oil and natural gas. The fracturing fluids are hydraulically injected into a well bore which penetrates the subterranean formation and are propelled against the formation strata by high pressure, forcing the strata to crack and fracture. Fracturing fluids can be thickened or gelled through the use of various chemical agents which act to increase viscosity or induce the gel formation. The viscosity of liquid hydrocarbon fracturing fluids can be increased by a variety of thickening agents, such as fatty esters and aluminum complexed fatty acids.
A situation is frequently encountered in well treatments where the bottom hole temperature or the environmental temperature at the locus of the fracture exceeds about 200xc2x0 F. (93xc2x0 C.). At such elevated temperatures, many types of fluids introduced into the formation to fracture the formation can undergo a reduction in viscosity, thereby losing their fracturing capability. For example, significant decreases in viscosity at temperatures exceeding 200xc2x0 F. (93xc2x0 C.) have been observed in the case of fracturing fluids prepared from hydratable polysaccharides or hydratable polyacrylamides. If an attempt is made to increase the viscosity of an oil-based fracturing fluid as it is made up and prior to pumping it into the subterranean location, the increased viscosity results in difficulty in pumping the fluid, and other problems also are encountered in handling the relatively high viscosity fluid at the surface. These difficulties are especially acute when solid propping agents are added to the fracturing fluid.
Therefore, there is a need for a well service composition useful as a fracturing fluid which would be stable at a temperature of about 200xc2x0 F. (93xc2x0 C.) or higher. It is desirable if such a fracturing fluid is stable at temperatures up to 300xc2x0 F. (149xc2x0 C.) to 350xc2x0 F. (177xc2x0 C.), or higher (e.g. 450xc2x0 F. (232xc2x0 C.)). It is also desirable that such a fracturing fluid be stable and useable at temperatures lower than about 200xc2x0 F. (93xc2x0 C.), for example as low as about 100xc2x0 F. (38xc2x0 C.). Moreover, it is also desirable that a fracturing fluid exhibits low frictional resistance to the flow of the fluid in the well bore conduit during fracturing, while maintaining a desired high viscosity in a fracture.
Liquid or solid phosphate ester-monoester copolymer compositions are disclosed. As one of many advantages, embodiments of these compositions offer effective, economical and environmentally compatible replacements for hydrocarbon-based, viscous well treatment fluids. The compositions comprise a base fluid and a monoester copolymer of styrene and maleic anhydride. The compositions can further comprise an alkyl orthophosphate ester. The composition can contain additional components such as a crosslinking agent capable of increasing the viscosity of the composition.
Methods of making and using the compositions are also disclosed. The compositions can be used for various uses, including injection into a subterranean formation.